Link to proper index.html.heex path in docs (#6037)

guides/testing/testing_controllers.md

@@ -49,7 +49,7 @@ def index(conn, _params) do
 end
 ```
 
-It gets all posts and renders the "index.html" template. The template can be found in `lib/hello_web/templates/page/index.html.heex`.
+It gets all posts and renders the "index.html" template. The template can be found in `lib/hello_web/controllers/post_html/index.html.heex`.
 
 The test looks like this: